1. Field of the Invention
The present invention relates to a control system for a vehicle having a continuously variable transmission capable of changing a gear ratio continuously.
2. Related Art
The continuously variable transmission can optionally set the gear ratio, so that it can set, when arranged on the output side of an engine for the vehicle, an engine speed resulting in a minimum fuel consumption required for each load such as throttle opening. Therefore the continuously variable transmission is a system effective for reducing the exhaust gas and improving the fuel economy.
An ordinary continuously variable transmission, as employed as a transmission of the vehicle, is constructed such that a transmission member for transmitting power is interposed between an input side rotary member and an output side rotary member while being in contact with these rotary members, to continuously change the gear ratio or the ratio between the speeds of the input side rotary member and the output side rotary member by continuously changing contact positions between the transmission member and the individual rotary members. One specific example is a belt type continuously variable transmission in which a belt is made to run on an input pulley and an output pulley, each of which has a variable groove width, or a toroidal type continuously variable transmission in which a tiltable power roller is sandwiched between a pair of discs having toroidal faces.
When such a continuously variable transmission is employed in the vehicle which is powered by an internal combustion engine such as a gasoline engine, the speed of the internal combustion engine can be controlled by the continuously variable transmission, and an output torque of the same can be controlled with a load such as throttle opening or fuel injection rate, so that the vehicle can be driven with minimum fuel consumption while satisfying a demand for driving force.
According to the continuously variable transmission, the gear ratio can be continuously changed, and the output of the prime mover such as the engine is continuously changed. In Japanese Patent Publication No. 3-72867 (JP-B-3-72867), therefore, there is disclosed a control system which is constructed to perform a highly efficient run by making effective use of the characteristics of the continuously variable transmission. In the disclosed system, a target driving force is determined from the amount of depression of an accelerator pedal and the vehicle speed to determine a target engine output, a target throttle opening and a target engine speed corresponding to the target driving force, and to determine a target gear ratio on the basis of the target engine speed and the vehicle speed. In the disclosed system, moreover, the target engine output and the target gear ratio are determined on the basis of an optimum fuel economy curve on which the fuel consumption rate is minimum.
In order to continue an autonomous rotation of the internal combustion engine such as the gasoline engine, a minimum speed has to be kept no less than a predetermined value. Such an idle speed control (i.e., ISC) is executed by controlling the throttle opening and the fuel injection rate so that an idling speed of the internal combustion engine may be a predetermined target value. In this case, the internal combustion engine acts as the prime mover for the entire vehicle, and the combustion of fuel is different depending upon temperature of the internal combustion engine. In order to keep the idle run, therefore, the control quantity of the internal combustion engine has to be corrected according to operating conditions such as load on accessories including an air conditioner and the temperature of the internal combustion engine.
In the aforementioned control system disclosed in Japanese Patent Publication No. 3-72867, however, a target output torque of the prime mover is determined on the basis of output demand such as accelerator depression, and the throttle opening and the fuel injection rate are so controlled as to equalize the output torque of the internal combustion engine to the target output torque. Even if there is need for a correction of the engine load based on the load on the accessories, which hardly appears as the output demand, therefore, the output torque of the internal combustion engine is not corrected to augment. As a result, it may become difficult to keep the idle run stably, when the load on the accessories rises.
An object of the invention is to provide a control system capable of executing an idle run properly in a vehicle provided with a continuously variable transmission.
In order to achieve this object, therefore, the invention is constructed to correct the output torque so that the idle run may be kept stable. More specifically, the control system of the invention is intended to control a vehicle having the continuously variable transmission mounted thereon.
According to an aspect of the invention, there is provided a control system for a vehicle including a continuously variable transmission, comprises: target output calculating means for determining a target output of a prime mover for achieving a target driving force; target output speed calculating means for determining a target output speed on the basis of the target output; gear ratio control means for controlling a gear ratio of the continuously variable transmission so that the output speed of the prime mover may be the target output speed; target output torque calculating means for determining a target output torque of the prime mover for achieving the target driving force; and load control means for controlling a load of the prime mover on the basis of the target output torque. This control system further comprises correction means for correcting a control quantity to control the load of the prime mover so that the output torque of the prime mover may have a sum of the target output torque and an output torque for keeping the idle run of the prime mover.
According to the invention, therefore, the target output torque of the prime mover is determined on the basis of the target driving force, and the load of the prime mover is so corrected that the output torque for keeping the idle run of the prime mover may have a sum of the target output torque and an output torque for keeping the idle run of the prime mover. As a result, the invention makes it possible to perform the control of the prime mover reflecting not only the target driving force but also the load necessary for the idle run, so that the idle run can be kept proper.
On the other hand, the correction means of the invention includes means for determining a corrected output torque for keeping the idle run of the prime mover and correcting the target output torque on the basis of the corrected output torque.
According to this construction, the target output torque, as determined on the basis of the target driving force, is corrected with the corrected output torque for keeping the idle run, and the load of the prime mover is controlled on the basis of the corrected target output torque, so that the idle run can be kept proper.
The control system of the invention can further comprise target speed correction means for correcting the target output speed on the basis of an accessory load.
In this construction, the target output speed determined on the basis of the target driving force is corrected on the basis of the accessory load. When the output torque is corrected to keep the idle run, therefore, the output speed of the prime mover is corrected accordingly. Therefore, the prime mover can be driven with a minimum fuel consumption by making effective use of the continuously variable transmission.
In this invention, still moreover, the target speed correction means includes means for determining an accessory load output for the accessory load and correcting the target output speed on the basis of the accessory load output.
In this construction, the target output speed of the prime mover is a value including the accessory load output, so that the output speed of the prime mover is optimized even in the presence of the accessory load.
In the invention, moreover, the correction means can include means for determining a corrected output to keep the idle run of the prime mover and correcting the target output on the basis of the corrected output.
With this construction, the target output providing the basis for determining the target output speed contains the corrected output for the idle run, so that the output speed of the prime mover can be proper for keeping the idle run and for driving the vehicle with minimum fuel consumption.
In the invention, further more, the correction means can include means for determining a corrected driving force for keeping the idle run of the prime mover and correcting the target driving force on the basis of the corrected driving force.
With this construction, the target driving force is corrected with the corrected driving force necessary for the idle run so that the target output speed and the target output torque of the prime mover are determined on the basis of the target driving force corrected. Therefore, the controls of the output torque and the output speed including the load for keeping the idle run can be executed to keep the idle run properly and to effect the drive with minimum fuel consumption.
In the invention, furthermore, the correction means can include means for changing a correction value for correcting the control quantity into a smaller value when the correction value exceeds a predetermined value during the running of the vehicle.
With this construction, the correction amounts of the target output torque, the target output speed, the target output and the target driving force are restricted when a correction value for correcting them exceeds a predetermined value. As a result, the prime mover can be prevented from being driven in a manner largely deviating from an optimum state.
The above and further objects and novel features of the invention will more fully appear from the following detailed description when the same is read in connection with the accompanying drawings. It is to be expressly understood, however, that the drawings are for the purpose of illustrations only and are not intended as a definition of the limits of the invention.